The present invention relates in general to an improved form of stub-support for a coaxial radio frequency transmission line. More particularly, the invention pertains to an improved stub-support for coaxial radio frequency transmission lines in which he support is more compact and has improved stiffness of support.
The use of quarter wavelength stubs for the support of the inner conductor with respect to the outer conductor of a coaxial line is known in the art. See, for example, U.S. Pat. No. 2,446,982 to Pound and U.S. Pat. No. 2,582,604 also to Pound. Both of these patents show how the bandwidth of such stub-supports is substantially increased by suitably lowering the impedance of the coaxial line for a quarter of a wavelength on either side of the stub-support. In this connection also refer to "Quarter-Wave Compensation of Resonant Discontinuities," by C. E. Muehe, IRE Transactions on Microwave Theory and Techniques (Correspondence), Vol. MTT-7, pp. 296-297; April 1959., and "The Application of a New Class of Equal-Ripple Functions to Some Familiar Transmission-Line Problems" by H. J. Riblet, Transactions of IEEE, Vol. MTT-12, pp. 415-421, July, 1964. These articles disclose closed expressions for the relationship between impedance of the coaxial line and the impedance of the stub for optimum performance as a function of the desired bandwidth. The prior theoretical treatment of this issue, however, has neglected the residual susceptances which are present at the junction of the stub and the coaxial line, perhaps because they are small in the round coaxial structures that have been considered heretofor such as found in U.S. Pat. No. 2,446,982 or U.S. Pat. No. 2,582,604, both to Pound.
Also described herein in FIG. 1 is a prior art structure in the form of a stub-support for a coaxial radio frequency transmission line having improved performance obtained by altering the dimensions of the outer conductor rather than altering the dimensions of the inner conductor. The improved performance is obtained by decreasing the dimensions of the outer conductor. In this regard, refer to FIG. 1 which is a sectioned perspective view of a compensated stub support. In FIG. 1 the coaxial radio frequency transmission line comprises an inner conductor and an outer conductor. The outer conductor comprises a pair of recessed channel members 1 and 2 while the inner conductor comprises a conductor member 3 which is of solid square cross-section. The inner conductor is supported within the generally square outer conductor by means of stub-supports which comprise oppositely-disposed inner conductors 4, outer conductors 5, and associated end walls 7. FIG. 1 also shows the transformer step 6 at the outer conductor. The electrical length of the stub-supports is, as depicted in FIG. 1, approximately 1/4 of a wavelength long at the middle of the useful operating frequency band of the coaxial structure. The shunt admittance of the stubs, as presented to the coaxial line, is zero at mid-band so that the stubs are essentially invisible to an RF signal traveling in the coaxial line.
In the prior art structure of FIG. 1 as well as in the structures described in the aforementioned Pound patents, the stub-supports are relatively large and cumbersome and provide support that is subject to a certain lack of stiffness or rigidity.
Accordingly, it is an object of the present invention to provide stub-support for a transmission line or the like device in which the stub-support is more compact.
Another object of the present invention is to provide an improved coaxial radio frequency transmission line in which the stub-support thereof is not only compact but also provides for improved stiffness of support for the coaxial line.
Still another object of the present invention is to provide a stub-supported transmission line device which is more compact and which thus enables the ready construction of more compact devices such as a coaxial hybrid device.